This invention relates to a method and apparatus for producing methane by hydrolysis of aluminum carbide. More particularly, aluminum hydroxide and/or aluminum oxide is reduced with carbon to form aluminum carbide which is hydrolyzed to form methane and aluminum hydroxide, and the aluminum hydroxide is recycled to the reducing step. Energy for the reduction step is supplied by combustion of low grade, and hence low cost, coal.
The prior art has disclosed the production of hydrocarbons, which may comprise methane, by hydrolysis of carbides of two or more metallic elements, and reformation of the carbides by reacting the recovered metallic elements with carbon which may be derived from coal.
U.S. Pat. No. 4,184,852, to J. J. Russ, discloses a method for making hydrocarbons containing at least about 85% methane by reacting water with metallic carbides which comprise at least one metastable carbide-forming metal and at least one stable carbide-forming metal. The hydroxides and oxides of the metals are recovered and used to make additional metal carbides. The use of both metastable and stable metal carbides is stated to be essential. Exemplary metastable carbide-forming elements include cadmium, zinc, barium, copper, zirconium, titanium, chromium, iron and lead. Stable carbide-forming elements include aluminum, manganese, calcium, magnesium, beryllium and boron. Carbon to form the carbides may be obtained from coal which may contain sulfur. Most of the sulfur is stated to be removable as a slag, while the remaining sulfur is in the form of hydrogen sulfide which is removed by scrubbing the hydrocarbons produced during formation of the metal carbides. The formation of carbides is carried out at a temperature of 1400.degree. to 2200.degree. F. (about 760.degree. to 1205.degree. C.), and the hydrolysis reactor is maintained at a maximum temperature of 350.degree. F. (about 175.degree. C.).
The Russ patent recognizes that the overall consumption of energy used to produce hydrocarbons is less than the energy value of the hydrocarbons produced. This is apparently based on a statement in the patent that stable carbide-forming elements give off energy upon formation of carbides, whereas metastable carbide-forming elements absorb energy upon formation of carbides. This is inconsistent with the fact that carbide formation of the exemplary stable elements is an endothermic reaction.
U.S. Pat. No. 4,310,334, to R. D. Waldron, discloses a process for producing hydrocarbons by hydrolysis of carbides of at least iron and manganese. Preferably a reactive metal such as calcium, magnesium, zinc and/or aluminum is also used. Alternatively, chromium, vanadium, or rare earth metals may be substituted partially. According to the patentee, aluminum carbide is not efficient in the production of synthetic fuel due to a low "net heat ratio", defined as "the ratio of heat of combustion of fuel gases produced to the heat of combustion of the carbide". There appears to be no definition in this patent of the term "heat of combustion of the carbide". Hence, the net heat ratio is believed to be a questionable criterion for the efficiency of the process. The mixed metallic carbides are formed in a synthesizer at a temperature of 1600.degree. to 2400.degree. F.
Waldron conducts the hydrolysis reaction in a conversion chamber maintained at 250.degree. to 600.degree. F., a range in which the temperature is high enough that most metal hydroxides will be dehydrated "and low enough that unwanted vapors, such as sulfur dioxide, hydrogen sulfide, etc., can be readily removed." Heat of reaction is used to generate steam in a heat exchanger "for use elsewhere."
Other U.S. patents relating to hydrocarbon production by hydrolysis of metallic carbides include 4,009,219; 4,184,852 and 4,317,659. An early disclosure of production of aluminum carbide is contained in U.S. Pat. No. 1,219,797.
While the broad concept of production of hydrocarbons by hydrolysis of metal carbides is well known, the prior art discussed above clearly teaches away from the use of aluminum carbide by itself as the metal carbide in the hydrolysis reaction. For reasons set forth in detail hereinafter, the present process avoids the alleged undesirability of using aluminum carbide as the sole metallic carbide.